Community Water Fluoridation Cessation and Dental Caries Incidence in South African Schoolchildren: A Target-Trial Emulation of Hygiene-Therapy Access Gaps

South Africa's phased withdrawal of community water fluoridation from several municipal supplies after 2015 created a natural policy discontinuity whose consequences for child dental health remain poorly characterised, in part because the before-after-control-impact designs used to evaluate such withdrawals are rarely powered to detect the caries increments that matter clinically. Power was estimated across 2000 replications per design cell for cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The analysis indicates that 54 clusters are required to detect a 5 percentage point difference at 80% power, a requirement that exceeds every cluster count examined and that most South African district-level evaluations could not meet. The results reframe the evidentiary problem of fluoridation cessation as one of design adequacy rather than of biological uncertainty, and they imply that hygiene-therapy access gaps will remain statistically invisible unless evaluation architectures are redesigned around the intraclass correlation actually observed in school-based caries surveys.